The invention is concerned with a method and circuit for controlling an alternating or three-phase current motor.
It is known that variable three-phase current motors strive to achieve through variable frequency and voltage an airgap flow which is as constant as possible, whereby the machine is optimally energized under all operational conditions. Even though the airgap flow of the machine can, in principle, be calculated as a controlled variable, it is considerably difficult using known methods. In the so-called "field oriented control", the energization of the machine is determined directly by measuring the induction in the airgap. To accomplish this, measuring coils for the electric flux are placed into the grooves of the machine or sound probes are placed in the airgap.
Such efforts are usually not justifiable. Normally, it is sufficient to keep the ratio constant between the supply voltage U and the rotating field frequency f--the so-called U/f ratio--and to equalize the load related voltage drop at the stator resistance of the machine through current resistance compensation, the so-called I.R compensation. Such methods are, however, very inexact since, instead of vector addition, only a quantitative addition of the voltage drop at the apparent impedance of the machine is carried out. In addition, the resistance of the stator winding of the machine is very dependent on temperature; for this reason, the I.R compensation is only suitable for specific operational conditions.
This frequency results in excessive no-load current in the machine when disconnected from a load.
For this reason there are known procedures which determine and control the U/f ratio through other motor parameters in relation to the load. The German Disclosure Brief 26 15 744 describes a procedure for controlling the U/f ratio in which the voltage drop at the main inductance of the machine is determined automatically under all operational conditions through an arithmetic element from sensing the terminal voltage. The stator resistance of the machine is entered as a constant into the approximation formula used, which is electronically simulated in the computer. It is also necessary to determine the applicable power factor. In this method of control the temperature effect is not eliminated and a considerable effort is required to determine the power factor of the machine. The same basic disadvantages are inherent in another known control method in accordance with the German Disclosure Brief 28 06 535, which only proposes a simplification of the mathematical formula for determining the supply voltage.
A procedure for controlling or regulating the speed of a three-phase motor is known through the German Disclosure Brief 29 39 090 on which the circuit factor of the machine is to be optimized for all load moments. The load moment, as the guiding value for the feedback control system, can affect the voltage at the main inductance of the machine only indirectly, in which case the previously described disadvantages apply and, in addition, it becomes necessary to simulate empirically the interconnection between the two values.